Most of traditional linear poly(ethylene terephthalate) (PET) resins of relatively low molecular mass and narrow molecular mass distribution have low melt strength at foaming temperatures, which are not enough to support and keep cells. An in-situ polymerization-modification process with esterification and polycondensation stages was performed in a 2 L batch stirred reactor using pyromellitic dianhydride (PMDA) or pentaerythritol (PENTA) as modifying monomers to obtain PETs with high melt strength. The influence of amounts of modifying monomers on the properties of modified PET was investigated. It was found that the selected modifying monomers could effectively introduce branched structures into the modified PETs and improve their melt strength. With increasing the amount of the modifying monomer, the melt strength of the modified PET increased. But when the amount of PENTA reached 0.35% or PMDA reached 0.9%, crosslinking phenomenon was observed in the modified PET. Supercritical carbon dioxide (ScCO2) was employed as physical foaming agent to evaluate the foaming ability of modified PETs. The modified PETs had good foaming properties at 14 MPa of CO2 pressure with foaming temperature ranging from 265 degrees C to 280 degrees C. SEM micrographs demonstrated that both modified PET foams had homogeneous cellular structures, with cell diameter ranging from 35 mu m to 49 mu m for PENTA modified PETs and 38 mu m to 57 mu m for PMDA modified ones. Correspondingly, the cell density had a range of 3.5x10(7) cells.cm(-3) to 7x10(6) cells.cm(-3) for the former and 2.8x10(7) cells.cm(-3) to 5.8x10(6) cells.cm(-3) for the latter.